During operation of an internal combustion engine, it is desirable to control the formation and emission of certain gases, such as the oxides of nitrogen (NO.sub.x). One method of achieving this result is the use of exhaust gas recirculation (EGR) which is a process whereby exhaust gases are selectively routed from the exhaust manifold to the intake manifold of the internal combustion engine. The use of EGR reduces the amount of NO.sub.x produced in a combustion chamber of the internal combustion engine. In particular, NO.sub.x is produced when nitrogen and oxygen are combined at high temperatures associated with combustion. The presence of chemically inert gases, such as exhaust gases, in the combustion chamber reduces temperature and inhibits nitrogen atoms from bonding with oxygen atoms in the combustion chamber thereby reducing NO.sub.x production.
A drawback with using EGR on a turbocharged diesel engine is that EGR cannot be used under conditions where the average pressure of air in the intake manifold is greater than the average pressure of exhaust gases in the exhaust manifold. Using EGR under these conditions would result in a net flow of air from the intake manifold to the exhaust manifold, rather than a net flow of exhaust gases from the exhaust manifold to the intake manifold needed to reduce NO.sub.x production. Therefore, an alternative method of supplying exhaust gases to the intake manifold must be used when the pressure in the intake manifold is greater than the pressure in the exhaust manifold.
Another disadvantage to using EGR is that the combustion products in the exhaust manifold contain a large amount of contaminants, such as soot and smoke. In particular, the intermittent combustion of fuel in reciprocating engines produces a large amount of soot and smoke. Recirculating this soot and smoke can foul the intake manifold, combustion chamber, and the valves and conduits necessary to recirculate the exhaust gas. Such fouling can potentially degrade the performance and usable life of the diesel engine.
What is needed therefore is an apparatus and method for advancing combustion products to the combustion chamber of a diesel engine which overcome one or more of the above-mentioned drawbacks.